Many signal processing circuits have a need to maintain known relationships between the signals traversing multiple signal paths. In practice, however, imperfections within a circuit can affect such relationships and adversely impact the performance of the circuit. One important example is that of an amplifier employing a linear amplification using nonlinear components (LINC) technique. In a typical LINC amplifier, a signal to be amplified is decomposed into two channel signals, both having an equal and constant envelope. It is known to use baseband inphase (I) and quadrature (Q) signal modulation of a radio frequency (RF) carrier signal to obtain the two channel signals. The decomposed signals are separately amplified then combined to form a linearly amplified signal. The LINC amplifier is attractive because the signals to be amplified have a constant envelope thereby enabling the use of efficient nonlinear radio frequency (RF) power amplifiers for amplification. As both signals are required to have equal amplitudes and proper phase relationships, it is important that the phase and gain for signal processing within each amplification path be balanced.
The promise of LINC amplifiers have yet to be fully realized in part because of the difficulty in developing circuits that maintain phase and gain balance for signals amplified using the separate amplification paths. For example, it may be desirable to achieve a level of amplitude signal accuracy of -65dBc for adjacent channel coupled power, and phase and gain resolution of within 0.5% for signals traversing the different amplification paths. Manufacturing imperfections within the amplification circuits make such balancing requirements difficult to achieve.
The prior art describes a variety of approaches to address such circuit imperfections. In one approach, phase and power measurements are made of RF signals and such measurements used to adjust for signal path imbalance. Adjustments typically involve phase and amplitude corrections along the RF signal path. Generally, phase measurements and adjustments tend to be complex and difficult to implement with extreme precision. In another prior art approach, a feedback loop is used which requires the sampling and processing of the RF signal to compare with the original baseband I and Q signals. This approach typically requires complex stability analysis to handle impedance load variations and other factors.
It is desirable to accommodate differences in circuits due to component imperfections and operating environment which can lead to unwanted variations in signal processing. This would enable implementation of LINC amplifiers and other circuits requiring the maintenance of a precise relationship between signals processed in different portions of a circuit. Therefore, a new approach to compensation for circuit imperfections is needed.